anyhow's dyn error backtrace disagrees with anyhow::Error::backtrace

[guswynn]

#![feature(backtrace)]
use anyhow;
use thiserror::Error;

#[derive(Error, Debug)]
enum Gus {
    #[error("hmm")]
    Other(
        #[from]
        #[backtrace]
        anyhow::Error,
    ),
}
fn main() {
    use std::error::Error;
    use std::ops::Deref;
    println!("{:?}", Gus::from(anyhow::anyhow!("main")));
    println!("{:?}", Gus::from(anyhow::anyhow!("main")).backtrace());
    println!("{:?}", anyhow::anyhow!("main").deref().backtrace());
}

results in

>printing the error with a backtrace
None
None

Looking briefly at the code, this appears to be at least semi-on-purpose, where the vtable for adhoc errors points to no_backtrace, but it links the backtrace back in somehow? can the ErrorImpl coercion do the same things as

anyhow/src/error.rs

Line 880 in 6833150

pub(crate) unsafe fn backtrace(this: Ref<Self>) -> &Backtrace {

?

(also, is there a reason the backtrace is only printed for the Debug impl, not the Display impl, should the docs be updated to reflect that?)

[dtolnay]

Updated repro:

#![feature(error_generic_member_access, provide_any)]

use std::any;
use std::backtrace::Backtrace;
use std::ops::Deref;
use thiserror::Error;

#[derive(Error, Debug)]
enum Gus {
    #[error("hmm")]
    Other(
        #[from]
        #[backtrace]
        anyhow::Error,
    ),
}

fn main() {
    println!("{:?}", Gus::from(anyhow::anyhow!("main")));
    println!("{:?}", any::request_ref::<Backtrace>(&Gus::from(anyhow::anyhow!("main"))));
    println!("{:?}", any::request_ref::<Backtrace>(&anyhow::anyhow!("main").deref()));
}

Other(main

Stack backtrace:
   …)
None
None

[dtolnay]

This is fixed as of thiserror 1.0.34 and anyhow 1.0.64.

Other(main

Stack backtrace:
   …)
Some(Backtrace […])
None

The last one is still None because you've written .deref() which specifically dereferences to the underlying Error impl that the anyhow::Error was constructed from, but the thiserror-generated Error impl has the correct backtrace now. It may be possible to do it for .deref() too but since your issue involves thiserror it doesn't sound like your issue was motivated by that case.